Hydrogen fluoride, or hydrofluoric acid (HF) is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry uses anhydrous hydrogen fluoride primarily as a liquid catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline and the HF alkylation process is responsible for producing a significant proportion of high octane gasoline components in the United States at the present time. In 1984, for example, the total U.S. HF alkylation capacity was 69,160 m.sup.3 (435,000 barrels) per day, using about 0.43 to 1.0 kg of HF per cubic meter of alkylate. In spite of the corrosive and toxic nature of hydrogen fluoride, the superior design, construction and operational precautions accumulated over years of experience in its manufacture and use have shown that HF can be handled safely, provided that its hazards are recognized and appropriate precautions taken. Although many safety precautions are taken there remains a concern for the potential danger of leaks.
An HF-alkylation acid composition is composed of about 88% HF, 6.5% acid soluble oils, 4% C3/C4 alkylate and 1.5% water. Generally, an alkylating plant is able to cope with minor spills of HF-alkylation acid caused by mechanical failures or corrosion, but in the unlikely event of a massive leak or spill of HF-alkylation acid from an alkylating unit, an aerosol cloud may form instantaneously. It is estimated that the cloud will be made up of 70 to 88% of HF aerosol droplets having a size in the range of 0.1 to 1.0.mu. and a surface tension of 8.62 dynes/cm at 18.2.degree. C. These droplets are very small compared to the droplets of water in a normal water drench and therefore a water drench requires large amounts of water for removing the acid spill.
Variables that control the size of droplets in a cloud include the charge on the particles, the surface tension of the liquid from which the particles are made, the influence of various solutes present in the HF-alkylation acid mixture as well as the mechanism of creating the aerosol droplets. Thus, the alkylation acid on escape will develop a cloud characteristic of its composition, the charge and size of the HF droplets and the surface tension of the droplets at the time of the leak.
A number of alternative methods to the simple water drench have been proposed. U.S. Pat. No. 4,210,460, for example, describes a method for treating an HF liquid spill by applying a quantity of an aqueous solution of calcium acetate to the spill in an amount equal to at least seven times the estimated volume of the spill, after which the spill is treated with powdered magnesium oxide and a pH indicator such as bromothymol blue. After the mixture reaches a persistent blue color, indicating a safe state, the spill is cleaned up mechanically.
At the 1982 Hazardous Material Spills Conference, Edward C. Norman of National Foam System Inc. reported the application of CHF-784 foam (a proprietary composition) to the contents of a damaged tank emitting an HF cloud after treatment with limestone. An immediate reduction in fume evolution was apparent after the foam application.
Gordon K. Braley, at the proceedings of the 1980 National Conference on Control of Hazardous Material Spills, in Louisville, Ky. on May 15, 1980 reported the treatment of relatively small amounts of controlled liquid spills of anhydrous hydrogen fluoride with high molecular weight polymers including polyacrylamide, polymethylmethacrylate, and polyvinyl alcohol. These materials applied in the form of a bead polymer formed a "skin" over the spill preventing fuming of the liquid. Polyacrylamide was deemed the most effective skin-forming agent.
Of the art cited above, only Edward C. Norman discusses an alleged successful treatment of a cloud containing HF. However, the composition used to treat such a cloud was not publicly disclosed.
U.S. Pat. No. 4,938,935 discloses a practical technique for altering or modifying the properties of the HF vapor cloud in order to make it more susceptible to knock-down by a water drench. According to the technique described in this patent, a proton acceptor which has multiple sites available for protonation is brought into contact with the HF acid to form multiply charged, highly polar species which modify the nature of the acid cloud and render it amenable to being treated with a water drench. The additive i.e. the proton acceptor, may be added to the alkylation acid as a permanent component of the inventory of acid or, alternatively, it may be added to the alkylation acid when necessary in the event of a release of acid from the unit.
U.S. Pat. No. 4,938,936 discloses a method of mitigating the effects of an HF leakage from a process unit by the use of a fighting agent such as sodium carbonate which generates a foam on contact with the acid and which also acts to neutralize the acid. The foam may be stabilized by the addition of surfactants and foam-forming agents such as hydrolyzed proteinaceous materials.
One problem encountered with the use of compounds which are added to the permanent inventory of alkylation acid, as distinct from being added when they are needed in the event of an emergency, is that they may react with the HF or be destroyed by it. Reaction products may interfere with the alkylation reaction and a total destruction of the additive will, of course, eliminate its utility for the desired purpose.